It is well known in the prior art to apply energy from electrodes to tissues, for various purposes, such as but not limited to, ablation, coagulation, necrosis, etc. For example, RF energy may be applied to pairs of electrodes in a bipolar mode of operation, wherein one of the electrodes is the cathode and the other is the anode.
One of the many uses of RF energy to ablate tissue is in the treatment of benign prostate hyperplasia (BPH). For example, one current technique, known by the commercial name as transurethral needle ablation (TUNA), involves the transurethral application of a medical probe having a pair of monopolar RF needle electrodes at its distal end. The probe is inserted into the urethra and advanced to a position adjacent the prostate. Thereafter, the RF needles are advanced to penetrate the urethral wall and access the prostatic tissue. A RF current is transmitted through each electrode and passes through the tissue to a grounding pad to form a necrotic region which is eventually absorbed by the body. Treating BPH with the TUNA technique is described, for example, in U.S. Pat. No. 5,366,490 to Edwards et al.
The use of RF electromagnetic energy in the thermal treatment of BPH has several limitations. For example, the use of monopolar RF electrodes presents problems in localizing thermal energy within a desired heating pattern within the prostatic tissue. Moreover, the heating patterns generated by the TUNA procedure with the monopolar electrode arrangement are nonsymmetrical. In addition, the flow of RF current from the monopolar electrodes to the grounding pad increases the potential of healthy tissue being subjected to thermal energy and destroyed. Furthermore, the monopolar electrode arrangement of the TUNA instrument is limited with respect to its ability to generate heating patterns of various shapes. Bipolar electrodes have also been used to treat BPH. For example, U.S. Pat. No. 6,016,452 to Kasevich describes a system that incorporates a bipolar or multipolar electrode array to create an electric field where the heat created is confined solely to a specific volume of the prostate gland and therefore the heated tissue is defined only by the electrode geometry. The bipolar electrode array provides a variety of three dimensional, symmetric heating patterns within the prostatic tissue depending on the relative electrode lengths and angular separation. The system provides precision tissue temperature and impedance measurements thereby enabling the surgeon to accurately predict heating pattern performance and tissue response to RF heating.